VSP is a measurement procedure in which a seismic source on or near the earth's surface transmits energy to a geophone secured at a desired depth to the wall of a borehole. This type of seismic recording differs from ordinary surface seismic reflection shooting primarily in the manner in which the geophones are positioned. In common midpoint shooting performed on land or in the sea, receivers are distributed laterally on the surface of the earth along the direction of profile. In VSP, geophones are positioned vertically to depths of sometimes several thousand feet and no receivers are generally positioned on the surface. Thus, the direction of geophone deployment differs by 90.degree. degrees in the two types of seismic recording.
In VSP, the seismic source is generally located as close to the borehole as possible. The idea is to send energy vertically downwards to reflecting interfaces so that the reflected energy will return vertically upwards to the borehole geophone located in the well. Data shot in this manner can be utilized to correlate sonic log data with conventional surface seismic data shot in the area of the well. However, this type of data has very little use in determining subsurface structure at any distance from the well since the energy travels substantially vertically.
There would be a number of advantages in using VSP to determine subsurface structure. Since the receivers are buried within the earth rather than on the surface, the receivers are less affected by distortion and noise which are generally present in surface seismic data. Also, since the receivers are so close to the reflectors of interest, the receivers have a greater ability to resolve the structure in the immediate vicinity of the borehole.
It is thus an object of this invention to provide a method for determining subsurface structure using VSP.